Metal surface treatment



? Se t. 1, 1953 R. POTTBERG 2,650,883

METAL SURFACE TREATMENT Filed Aug. 9, 1949 A T B 0 SURFACE v- HEATSCALED TREATMENT W PRODUCT 5 5 CHEMICAL MECHANISM FOR TREATMENT GRIPPINGAND DRAWING STRIP THROUGH'W'AND'B FIGZ SURFACE TREATMENT HIHH HIIHIIHHHHHI ///A 'STBIPETQE cou'mun 0 mm 7 MAJORITY OF CRACKIING OOGURSOVER THIS ANGLE INVENMR.

Patented Sept. 1, 1953 METAL SURFACE TREATMENT Rolfe Pottberg, LochRaven, Md., assignor to Freeport Sulphur Company, New York, N. Y., a

corporation of Delaware Application August 9, 1949, Serial No. 109,380

1 Claim.

This invention relates to the conditioning of metal surfacesparticularly in connection with the removal of oxide films formed, forinstance, by strip or sheet rolling operations at elevated temperatures.

The object of the invention is to provide a method of modifying suchoxide coatings so that subsequent pickling or other treatments will berendered more promptly and efficiently effective in removing thesecoatings.

Other objects of the invention, particularly in the special proceduresand apparatus employed in the treatments, will appear from the followingspecification taken in connection with the accompanying drawing inwhich:

Fig. l is a schematic outline of a surface cleaner system employing theinvention;

Fig. 2 is a diagrammatic view of the surface treatment; and

Fig. 3 is an enlarged view of a portion of Fig. 2.

In the system outlined in Fig. 1 a metal strip having an oxide surfacelayer 6 is passed through the primary surface treatment stage A and onto the secondary surface treatment B preferably by drawing means C.Stage A involves the application to the strip of physical forces in amanner concentrating this effect on the surface and in particulardisrupting the continuity of the oxide films so as to condition them forready response to subsequent removal treatment indicated at B. TreatmentA may be at room temperature or any temperature substantially below thatat which such oxide films are formed.

As diagrammed in Fig. 2, the primary surface treatment at A impressesthe metal strip by rollers l under adjustable pressure at 8 so that theforces applied to the strip surfaces 6 may be adjusted and regulated.Preferably the rollers l are idlers, the strip being drawn through thisstage in any desired manner. The pressure applied by rollers Icompresses the strip 5 thinning it somewhat and so causing an angle bende and rippling Ill of the metal surface in advance of the roll contactas illustrated, for instance, in Fig. 3. The pressures involved andsurface distortions are not suflicient to clear the oxides from thesurface and substantially no oxide is removed here.

I have found that a much improved over-all removal and descaling isattained by disintegrating the surface oxide layers in situ under theaction of this bending and rippling effect and completing the actualremoval at the later stage B. Such preliminary modifications of thephysical nature of the surface may be attained at no loss of time andwhile the strip is en route to the removal stage B. Here, at stage B,the time of treatment is greatly reduced by the modified structure ofthe surface films and this permits a higher rate of feed, or converselyshorter length of the removal stage B. Such shorter timing at stage B,usually a chemical cleaning, also reduces the action on the metal itselfand minimizes the losses involved.

To attain the desired condition of the surface the compression effect isregulated to limit it to a .disintegrating of the scale without removingit, thus retaining the surface oxides while developing within themmultitudinous microscopic fractures honeycombing the entire film andreacting right down to the metal itself. Any concomitant permanentthinning of the strip from thickness d to thickness b (Fig. 3) is atmost a small percentage, 2% to 4% being characteristic, and in somecases the reduction is imperceptible, the elastic limit of the materialnot having been substantially exceeded. Apparently the invisible networkof microscopic fractures, left in tightly closed position by passagebetween the rolls, serves as a maze of latent paths of attack and entryfor the usual pickling solutions and it has been found that wettingagents are not necessarily added.

In attaining this modification of the surface oxide film, the bend 9 andthe radius of the crest of the metal ripple or Wave [8 at the surface ofthe metal entering the rolls are functions of roll diameter and are ofan entirely different degree of magnitude in comparison with the radiiachievable by bending the entire shape over a pulley, for instance. Thesmaller the diameter of the roll l the less the pressure as determinedby the percentage reduction in thickness will have to be. This is ofpractical significance for two reasons. By the use of very smalldiameter work rolls bearing upon heavier back-up rolls, the amount ofcold working to which the bod of the metal section must be submitted maybe held to a minimum and, in some cases, the portion of the crosssection in which the elastic limit is exceeded may be confined to theimmediate metal surface. Secondly, the amount of work which must be doneon the metal becomes so small that it is often possible to pull materialsuch as a rod or strip through the rolls by use of the existingtraversing equipment without the necessity of driving the rolls.

It has been found advantageous to employ the rolls 7 having diameters inthe range between 5 and 50 times the thickness of the material being 3treated and effecting a reduction of less than in the thickness of themetal between the rollers.

The following examples of the working of the process will make clear thevery radical change in the character of the oxide film under the actionof the surface rolling treatment:

Example I.- H ot.rolled alloy steel strip, in thickness containing 12%of chromium, required two hours of treatment in a 10% sulphuric acidsolution at 150 F. to effect all scale removal. Severe acid attack ofthe base metal o'ccurred. The same strip, passed between 3" diam terrolls set to effect a 2% thickness reduction required only 10 minutes inthe same acid solution to effect all scale removal, there beingnegligible attack on the base metal.

Example II.Box-annealed stainless steel plate, in thickness, requiredthe following procedure to effect scale removal: minutes in fusedcaustic alkali made oxidizing by addition of oxidizing salts, followedby water quench, followed by 10 minutes in5% sulphuric acid at 160followed by 5 minutes in 15% by volume nitric acid solution at 150 F;the whole of the above procedure having to berepeated again beforecomplete descaling was effected, When the same strip was given a 4%reduction by passing between rolls as above described, it was possibleto completely clean it according to the following procedure; 5 minutesin the salt bath above described, followed by a water quench, followedby two minutes and'one minute respectively in the acid solutions abovecited.

Example III.Scaled, high carbon steel strip required lz minutes toeffect complete descaling in an 8% sulphuric acid solution at 150F.After a thicknes'sreductionof 2%, the same strip re- .8 is'achievedjwhichf results in the production of successive f minutely'juxtaposed, microscopic fractures. These fractures'or cracks areimmediately closed again when they pass between the jrolls buttheynowrepr'e'sent mechanical discontinuities in jthes'cale which serve aslines of ac- ,oess forsubsecluent liquid treatment. The roller actionhas no "physical effect upon the scale film other than sometimes torender it somewhat shinier in appearance and results in substan- "tiallynoscale removal. By its use,however, the

area of scale surface'available to be attacked is very greatlyincreased. Furthermore, the added areas are inlarger part at rightangles to the scale surface and extend to the base metal. This has theeffect of exposing to chemical attack the less resistant oxidecomponents occurring in the layers nearer the base metal. Such deeperoxide films are generally more reacting particularly with acids due totheir lower state of oxidation. The accessibility of the base metalitself at the bottom of the fractures also results in acceleration ofthe electrolytic influences tending to aid in removal of scale films byacid action.

The amount of cold working produced in the body of the metal is veryslight in comparison, for

instance, with scale-breaking treatments which both by stretching and bybending operate to impart'a substantialdegree of permanent stretchthroughout the metal, the present method concentrating the majority ofbase metal distortion at the surface.

I claim:

In the treatment of. a metal article of sheet, plate or strip formhaving an adherent surface film of scale thereon, subjecting thescale-coated article at a temperature below that of scale formation torolling pressure between opposite rollers having their axes in a planenormal to the article surfaces and intersecting them at substantiallythe ar ea'of contact'of the rollers therewith, the diametero f-atleast'one of said'rollers being more than five times and less than fiftytimes the thicknessof the strip, and f orcin'gjsaid rollers against thearticle surfaces'under sufilcient pressure to develop 'a ripple in th'escalecarrying surfacein advance'of said rolling pressure'cont'act,progressively disintegrating the-said film of scale by saidroller'oflimiteddiameter and preserving the'disint'egratedfilm in place on thearticle surface and simultaneouslyeffecting a reduction of less than 10%in the thicknessof the metal between the rollers, delivering the articlewith the disintegratedscale film-adhering thereto, and subsequentlytreating thesaidj'adhering disintegrated scale film with a fluidreacting therewith toloosen'and remove it.

ROLFE POTTBERG.

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